The present invention relates to a heat exchanger for cooling reaction gas in an ethylene plant. Within an ethylene plant, pyrolysis or ethylene cracking or disassociation furnaces form the precursors or key components for the manufacture of the base materials ethylene, propylene, butadiene, and others for the plastics industry. Used as starting material are saturated hydrocarbons, principally ethane, propane, butane, natural gas, naphtha, or gas oil. The conversion of the saturated hydrocarbons into unsaturated hydrocarbons takes place in the cracking tubes of the cracking furnace, and in particular at inlet temperatures of 500-680° C. and discharge temperatures of 775-875° C. in a pressure range of 1.5-5 bar.
In subsequent reaction gas coolers disposed at the outlet of the cracking furnace, the unsaturated hydrocarbons, the so-called reaction gases, are cooled from 775-875° C. to approximately 350-450° C. accompanied by the formation of high and low pressure vapor. In this connection, the “cooling water” has a boiling temperature at an appropriate pressure. The cooling takes place due to the phase transition from liquid to gaseous. The steam is utilized in the ethylene plant, for example for steam turbines.
The cooling of the reaction gas, accompanied by the formation of steam, takes place either in single-stage systems, whereby the entire cooling to about 350-450° C. takes place in only a singe reaction gas cooler, or in two-stage systems, whereby a cooling is effected in stages in two reaction gas coolers that are disposed one after the other; for example, in the first stage from 875° C. to 550° C., and in a second stage from 550° C. to 350° C. The reaction gas coolers have the corresponding designation primary cooler and secondary cooler.
In addition, a further cooling of the reaction gas is effected in boiler water supply preheaters not only in the single-stage system but also in the two-stage system. Here, steam is no longer generated, rather, the “cooling water”, the boiler supply water, is preheated as close as possible to the boiling temperature for the primary and secondary coolers. The supply of the preheated boiler supply water to the primary and secondary reaction gas coolers is effected indirectly via a steam drum, in which the boiler supply water is heated to the boiling temperature.
A reaction gas cooler is known from EP 0 272 378 B1 according to which the reaction gas is cooled in a first cooling stage, which represents an evaporator, by boiling water, and is cooled in a second cooling stage, which represents a superheater, by steam. As is customary, an additional cooler is disposed downstream of the reaction gas cooler in which the reaction gas is cooled down further by feed water. With a variant of the reaction gas cooler known from EP 0 272 378 B1, the evaporator and the superheater are disposed in a common casing and are separated from one another by a partition that prevents the cooling agent from flowing over from one cooling stage into the other cooling stage.
It is an object of the present invention to provide a heat exchanger for cooling reaction gas, which heat exchanger includes two partial chambers within a common jacket, in such a way that the cooling within the partial chamber disposed on the gas inlet side for the reaction gas is more effective, and that the structural buildup is reduced.